[2] Washington Township is located in northeastern Butler County and contains the unincorporated communities of Hilliards, Argentine, Annisville, Whiskerville, Parsonville, and North Washington.

Slippery Rock Creek rises in the northern part of the township.

[3] As of the 2000 census,[5] there were 1,419 people, 511 households, and 396 families living in the township.

There were 511 households, out of which 37.2% had children under the age of eighteen living with them; 64.0% were married couples living together, 10.0% had a female householder with no husband present, and 22.5% were non-families.

Of all of the households documented, 18.0% were made up of individuals, and 7.0% had someone living alone who was sixty-five years of age or older.